The present invention relates to an optical disc.
FIG. 1 is a perspective view of an optical disc 4 according to the prior art. In the drawing, reference numeral 1 denotes a disc-shaped substrate which is formed of polycarbonate or the like and has a centering hole 1a (whose diameter is 15 mm, for example) in the center. 2 denotes an information recording layer which is formed concentrically at a specified radial area on the disc substrate 1 and which is used for recording and reproducing information by an input-output device, not shown. 3 denotes a disc of magnetic material whose outer diameter is smaller than the inner diameter of the information recording layer. The optical disc 4 is formed by bonding one disc 3 to the upper face of the disc substrate 1 with an adhesive, and further by bonding another disc 3 to the lower face thereof so that the substrate is sandwiched between a pair of discs 3.
FIG. 2 is a sectional view of the optical disc 4 mounted on an optical disc driver, in which the disks 3 are attracted by a magnet 5a mounted on a disc turntable 5. The optical disc 4 is rotated by turning the disc turntable 5. Under such condition, information is written in the information recording layer 2 or read out therefrom by irradiating the layer with a laser beam.
According to the aforementioned prior optical disc, there is a difference between the thermal expansion coeffcient (7.times.10.sup.-5 mm/.degree.C.) of the disc substrate 1 and that (1.times.10.sup.-5 mm/.degree.C.) of the discs 3 of magnetic material, and therefore a problem exists in that distortion occurs in the disc substrate 1 whose thermal expansion coefficient is larger due to temperature change in the internal section of the optical driver. The distortion increases variations in the birefringence of the laser beam in the disc substrate irradiated when reading out information, thereby increasing noise when reproducing the information.